Generally speaking, optical memory cards are plastic cards of a stipulated size configured in such a manner that their recording area may be recorded on to and reproduced with the aid of a laser beam.
FIG. 10 shows the recording area 102 of a conventional optical memory card 101. As may be seen from the enlarged drawing, the recording area 102 has data recording tracks 1 and guide tracks 2 arranged alternately on it. The data recording tracks 1 are the parts which serve to record data, while the guide tracks 2 are formed at a specified interval, which is required for the purpose of tracking control whereby the irradiation position of the laser beam used for recording and reproducing is stabilized.
Here, the reflection factors of the data recording tracks 1 and the guide tracks 2 differ, and a comparison between the two shows that of the data recording tracks 1 is high, while that of the guide tracks 2 is low.
The recording pits 3 are sections with low reflection factors which are formed by a laser beam within the recording tracks 1 at the time of recording. The presence or absence of these recording pits 3 corresponds to the digital codes 0 and 1.
FIG. 11 is a diagram illustrating the configuration of the optical system which records and reproduces information for the optical memory card 101.
In FIG. 11, a laser beam emitted from a laser diode 103 is rendered into a parallel beam by a collimator lens 104, is incident upon a diffraction grating 105, and is split into innumerable light rays.
The light rays which have been split with the aid of the diffraction grating 105 are directed on to a beam splitter 106, the zero-order beam being used as the main beam for recording and reproducing data, while the .+-.-order beam is used as a sub-beam for tracking control.
Part of the rays which are incident upon the beam splitter 106 are reflected in a 90.degree. direction and directed on to a power monitor 114 which monitors the strength of the light source. The remaining rays pass through the beam splitter 106 and proceed straight ahead by way of a reflecting mirror 107 and objective lens 108 to converge as three spots on the recording area 102 of the optical memory card 101.
Reflected off the recording area 102, the rays are rendered into a parallel beam once more by the objective lens 108, pass through the reflecting mirror 107, and are reflected in a 90.degree. direction by the beam splitter 106. They then proceed by way of a collimator lens 109 and concave lens 110 to assume a long focal length and be directed on to an edge mirror 111.
This edge mirror 111 is a reflecting mirror which is located so as to screen half the rays, so that they are divided into half the main beam along with the +-order light of the sub-beam on the one hand, and the remaining half of the main beam along with the --order light of the sub-beam on the other.
Having been divided by means of the edge mirror 111, the half of the main beam and the +-order light of the sub-beam proceed straight ahead to form an image on an photosensor 112, while the remaining half of the main beam and the --order light of the sub-beam are reflected in a 90.degree. direction to form an image on an photosensor 113.
The rays which form images on the photosensors 112 and 113 are here converted into electric signals.
It should be noted that the main beam (0-order light) 7 and the sub-beams (.+-.-order light) 8a, 8b, having passed through the objective lens 108, are directed on to the recording area 102 as illustrated in FIG. 10. The recording pits 3 are formed by this main beam 7 in the center of the data recording track, which is to say equidistantly from the two adjoining guide tracks 2. For this reason, the main beam 7 must always be directed on to the center of the data recording track 1. Consequently, tracking control is implemented with the aid of the sub-beam 8 (or the sub-beams 8a and 8b) and the guide tracks 2.
The main beam 7 and the sub-beams 8a and 8b are always incident on the recording area 102 at a prescribed interval. When the main beam 7 is incident in the correct position, half of each of the sub-beams 8a and 8b is incident upon the data recording track 1, and the remainder upon the guide track 2.
Inasmuch as the edge mirror 111 causes the reflected light of the two sub-beams 8a and 8b to form images on separate photosensors (e.g. the sub-beam 8a on the photosensor 112, and the sub-beam 8b on the photosensor 113), it is possible to calculate the reflection factor of the positions upon which the respective sub-beams are incident.
If each of the beams 8a and 8b are incident in the correct position, the reflection factors of the positions in which they are incident are equal, as therefore are the respective strengths of the reflected light.
However, if the irradiation position of a beam is displaced to the left or to the right, the difference between the respective strengths of the reflected light of the sub-beams 8a and 8b appears as a positive or negative value. This difference is converted into an electric signal and fed back as a tracking error signal. This drives the objective lens 108 in a horizontal direction, controlling it so that it assumes a state where this tracking error signal is 0, namely where the main beam 7 is incident in the correct position.
FIG. 12 illustrates the logical data configuration of the recording area 102. In the recording area 102 are recorded, apart from a sector 120 which is the part where data is recorded, a lead-in 121 which allows the device for recording and reproducing the optical memory card (not shown in the drawing) to achieve bit synchronization during recording and reproduction, a SYNC mark 122 which facilitates frame synchronization, a BOS 123 which is necessary in order to detect the position at which the sector 120 begins, and other information.
In the lead-in 121, not only is the SYNC mark 122 recorded, but the recording pit 3 is formed for each recording interval (synonymous with the digital code 1 1 1 1 . . . ).
The device for recording and reproducing the optical memory card (not shown in the drawing) has a device for generating a synchronizing signal (not shown in the drawing). The memory card 101 is scanned at a scanning rate such that the synchronizing signal output from the device for generating a synchronizing signal matches the initialization signal of each bit which is detected by scanning the lead-in 121, and bit synchronization is achieved by maintaining this scanning rate.
A modulation system wherein a synchronizing signal is included in the recorded data is sometimes employed in order to ensure that the achieved bit synchronization is maintained, and the synchronizing signal is extracted from the signal detected during reproduction.
Here, the SYNC mark 122 is formed by arranging recording pits 3 in a pattern which is not generated by modulation, and this is used so that the device for recording and reproducing the optical memory card (not shown in the drawing) can acquire the frame signal.
The frames are sequences of bits when the signal processing circuit within the device for recording and reproducing the optical memory card (not shown in the drawing) is processing signals. The device for recording and reproducing the optical memory card (not shown in the drawing) has a counter (not shown in the drawing) for achieving frame synchronization. When it has counted up to the number of bits which constitute a frame, it outputs a frame synchronizing signal and clears the counter value.
Here, the SYNC mark 122 is recorded at the beginning (or end) of the frame. The device for recording and reproducing the optical memory card (not shown in the drawing) scans the optical memory card 101. When it detects the SYNC mark 122, it clears the counter (not shown in the drawing) for achieving frame synchronization, ensuring that frame synchronization is achieved.
When a track holding mistake occurs in the conventional method for controlling the tracking of an optical memory card as described above, and vibration of the actuator or an external shock of some kind causes the main beam while scanning a data recording track 1 to migrate to a different data recording track 1, control is implemented in such a manner that the main beam tracks the center of the recording track 1 to which it has migrated.
In such a case, a problem occurs in that if the track to which the main beam has migrated is a data recording track 1 on which data has already been recorded, the data is overwritten and lost.
In order to prevent the overwriting of data, it is vital to distinguish between a data recording track 1 which has already been recorded and one which has not. It has been suggested that a modulation system might be adopted whereby recording pits 3 are formed within a specified interval even if the data recorded is a succession of 0 signals (not forming recording pits), but efficiency is poor. Moreover, even if this system is adopted, it is difficult to prevent overwriting altogether because it occurs before there is time to detect that the track to which the main beam has migrated is a data recording track 1 on which data has already been recorded.
Meanwhile, conventional devices for recording and reproducing optical memory cards have a circuit for generating a synchronizing signal in order to achieve bit synchronization. They also have an actuator and control circuit whereby it is possible to maintain a constant scanning rate in order to ensure that the bit synchronization achieved by scanning the lead-in 121 is maintained. This means that if the action of recording and reproduction is to be speeded up, it is necessary to improve the accuracy of the scanning rate, including the accuracy of the actuator.
Moreover, not only are the lead-in 121 and SYNC mark 122, which are vital so that the device for recording and reproducing the optical memory card to achieve bit and frame synchronization, recorded on the conventional optical memory card, but a modulation system is adopted whereby a synchronizing signal is included in the data which is recorded for the purpose of ensuring that bit synchronization is maintained. Thus, the adoption of a modulation system such as allows data recording tracks 1 which have already been recorded to be distinguished from those which have not results in problems of lower recording efficiency and a reduction in the amount of data which can be recorded on one optical memory card.